Power Systems

Lesson Objectives When you finish this lesson you will understand: Control of the high electrical power systems, transformer control Timing functions Calculation of the heat Troubleshooting Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.Do on-line workbook 5.Do Homework Keywords Power Systems, Single Phase AC, Single Phase Rectified, Three Phase, Three Phase Rectified, Frequency Changer, Inverter, Transformer, Tap Switches, Saturation, Hysteresis, Reactance, Inductance, Resistance, Impedance, Power Factor, Duty Cycle, Timing Controls, Synchronous, Phase Shift Heat Control, SCR, half wave rectified, Full Wave Rectified

Machines and Equipment Cooling System Mechanical System –Force generation system Pneumatic/Hydraulic system Motor or foot-operated system –Electrode application system Rocker arm or pinch type Press or direct action type Portable system Electrical System –Single-phase system –Single-phase rectified system –Three-phase rectified system –Frequency changer system –Frequency inverter system

Block Diagram of Single-Phase Spot Welder Spot Weld Main Power Line Contactor

Configurations for Spot Welding Transformers (a) Stacked, Shell Type (b) Stacked, Core Type(c) Wound Type

W. Stanley, Resistance Welding McGraw-Hill, 1950

Parallel/Series Tap Switches EsEs EsEs EsEs EsEs VpVp S1S1 S1S1 S2S2 S2S2 P2P2 P2P2 P1P1 P1P1 VpVp (a) Parallel (b) Series

Transformer Tap Switch EsEs VpVp

Applied Magnetic Field Induced Magnetic Filed Saturation Induction Remnant Induction Coersive Force Structure & Properties of Materials, Wiley, 1967

RpRp XpXp RsRs XsXs X = reactance in Ohm L = inductance in Henry f = frequency in Hertz Z = impedance in Ohm R = resistance in Ohm

Power Factor DC Current AC Current Ohm’s Law I= Current V= Voltage R= Resistance Z= Impedance X= Reactance Frequency of AC Geometry of Secondary Magnetic Material Power = VI =KW (kilowatts) Power = KVA

Power Factor P V P I I V (a) Purely Resistive Load (b) Resistive and Reactive Load P av = V av I av cos(  )

Power Factor = Cos If: I=10,000 amps V=10 volts Kva=100 Kw=70.7 If: R=X pf = 70.7%

Variables Affecting Power Factor Machine Design Single-Phase versus Three-Phase Current Frequency Reactance/Resistance of the Secondary Loop Magnetic Material in the Throat

Resistance Welding Manual, RWMA, 1972 R X Increased Angle Decreased Power Factor Increased Throat Length

Ferrous Material in Throat Resistance Welding Manual, RWMA, 1972 R X Reduced Power Factor

Power Factor Correction Capacitors Contactors (Switched Simultaneously) High Impedance DC Charging Circuit AC Power Supply AC Power Supply Welding Contactor Normally-Closed Contactor Discharging Resistor Tuned Capacitor (a) Shunt Capacitors (b) Series Capacitors

Duty Cycle is the power level at which a transformer can operate for some fraction of time without overheating Percent Duty Cycle Percent Rated Load 50% 100%

Timing Controls Non-synchronous Controls Synchronous Controls Microprocessor Control = Synchronous At what point in time do we close the contactor

Electronic Control Res. Weld, McGraw-Hill, 1943 Synchronous Timing Accuracy

Transient Currents with Non-Synchronous Timing Electronic Control Res. Weld, McGraw-Hill, 1943

Electronic Control Res. Weld, McGraw-Hill, 1943

Control of Secondary Current Transformer Tap Setting (Turns Ratio) Phase Shift Heat Control

Rules for SCR Conduction can conduct when anode is + with respect to cathode once initiated, can only be stopped by removal of anode voltage initiated by a pulse on ignitor Cathode Anode Gate

Welding Handbook

“Idealized” Phase Shift Heat Control (a) Current Controlled by Tape Settings (b) Current Controlled by Phase Shift Setting In Actual Case, Current can not instantaneously change

Firing Angle Hangover Angle Related to R/L ratio Actual Condition Androvich “Resistance Welding Constant Current Heat Control”, AWS Sheet Metal ConfV, 1992

I RN = Normalized Available RMS Current Androvich “Resistance Welding Constant Current Heat Control”, AWS Sheet Metal ConfV, 1992

Tsai, Experimental Study of Weld Nugget Expan, Paper B1, Sheet Metal Welding Conf V, AWS, 1992

Link to: Phase Shift Heat Control Demo

Portable Trans Guns Transformer Integral with Gun Thus smaller in Size Nedorezov, J “Using Portable Transguns for Resistance Spot Welding”, Practical Welding Today, Nov-Dec 1997

Machines and Equipment Cooling System Mechanical System –Force generation system Pneumatic/Hydraulic system Motor or foot-operated system –Electrode application system Rocker arm or pinch type Press or direct action type Portable system Electrical System –Single-phase system –Single-phase rectified system –Three-phase rectified system –Frequency changer system –Frequency inverter system

Single-Phase Rectified System Center-Tapped Transformer Bridge-Type Rectifier Primary Current Secondary Current

Single-Phase Rectified System Advantages Higher Power Factor More Uniform Heat Flow Pattern Less pf Loss by Material in Throat Disadvantages Expensive Need Heavy Duty Diodes One Electrode Wears Rapidly Load on one side of Three Phase Line Balanced 3 phase hook-up needed A B C

Machines and Equipment Cooling System Mechanical System –Force generation system Pneumatic/Hydraulic system Motor or foot-operated system –Electrode application system Rocker arm or pinch type Press or direct action type Portable system Electrical System –Single-phase system –Single-phase rectified system –Three-phase rectified system –Frequency changer system –Frequency inverter system

Three-Phase Rectified System Half Wave Full Wave Primary Current Secondary Current 10% Heat 100% Heat

120° Half Wave Full Wave * * **

Spinella, D., “Al RSW: Capital and Operating Costs”, AWS Xsheet Metal Conf, 1996

Comparison of Two PF on Three Machines High Ripple Medium RippleLow Ripple Roth, “Alternating Current Vs DC in Resistance Welding” AWS Sheet Metal Conf IV, 1990

Both Single Phase and 3 Phase DC is best in low power factor (high inductive) circuits providing as much smoothing as possible Phase shifting will increase ripple (more in single phase). Operate on the highest phase shift and lowest tap setting possible Comparison of Two PF on Three Machines

Roth, “Alternating Current Vs DC in Resistance Welding” AWS Sheet Metal Conf IV, 1990 Need to get comparison with 3 phase DC circuit 2 as 100%

Advantages High Power Factor Uniform Heat Flow No need to balance hook-up No pf loss by material in throat Disadvantages Expensive Need Heavy Duty Diodes One Electrode Wears

Machines and Equipment Cooling System Mechanical System –Force generation system Pneumatic/Hydraulic system Motor or foot-operated system –Electrode application system Rocker arm or pinch type Press or direct action type Portable system Electrical System –Single-phase system –Single-phase rectified system –Three-phase rectified system –Frequency changer system –Frequency inverter system

Controller A Controller B Controller C Frequency Converter

Frequency Changer System (a) Frequency-Changer (b) Rectified

Advantages Power Factor > 98% Balanced Loading Balanced Electrode Wear Disadvantages Cost Control Problems

Machines and Equipment Cooling System Mechanical System –Force generation system Pneumatic/Hydraulic system Motor or foot-operated system –Electrode application system Rocker arm or pinch type Press or direct action type Portable system Electrical System –Single-phase system –Single-phase rectified system –Three-phase rectified system –Frequency changer system –Frequency inverter system

Frequency Inverter System DC Rectifier AC 60 Hz Oscillator AC HF 400, 600, or 1200 Hz

Advantages Disadvantages High Power Factor > 98% Less Core Material Lighter & Smaller – suitable for robots Cost Newer Process – Still Under Development

Duty Cycle